Loops in the active topology of bridged networks can lead to severe problems including unicast frame duplication, multicast frame multiplication, and address table nonconvergence. The inability to tolerate active loops is a fundamental limitation to the topology of bridged networks. Such loops must be eliminated to ensure proper network operation. Generically speaking, two approaches have been taken in the past to eliminate loops: manual and automatic configuration.
Manual configuration simply means to configure the passive topology of the network so as to avoid loops. However, it is not always possible or even desirable to manually configure a loop-free topology. Manual configuration of a loop-free topology is generally only acceptable for small networks. Additionally, even in small networks link redundancy may be necessary and therefore small networks may have loops.
Networks may be designed using a plurality of distinct topologies, that is the entities in the network may be coupled together in many different ways. Referring to FIGS. 1, there is shown an example of “ring” topology. A ring topology is a network configuration formed when “Layer 2” bridges are placed in a circular fashion with each node having two and only two ports belonging to a specific ring. FIG. 1 shows a single ring having nodes 110, 120, 130, 140 and 150 connected by communication paths.
Ring topologies shown in FIGS. 1 and 2 present Layer 2 traffic looping problems. As illustrated in FIG. 1, in a single ring topology, data traffic can circulate around in either direction past their origination and thus create repetition of messages. For example, data traffic may originate in node 110, travel counter-clockwise in the ring, pass node 120, 130, 140, 150 and return to node 110; this is called a loop. Loops are highly undesirable because data frames may traverse the loops indefinitely. Furthermore, because switches and bridges replicate (i.e., flood) frames whose destination port is unknown or which are directed to broadcast or multicast addresses, the existence of loops may cause a proliferation of data frames that effectively overwhelms the network.
To prevent looping, one of the paths in the ring is blocked, as shown in FIG. 2, by blocking data traffic in one or more of the ring ports; in this case, port 208 is blocked. The port is deemed to be in a “blocking” state, in which it does not learn or forward incoming or outgoing traffic.
Automatic configuration eliminates loops in the active topology of the network by means of a special computer protocol. These protocols are called are management protocol which run over the switch algorithms and create only one active path at a time to avoid loops. One such example of a protocol used for eliminating loops in a network is a Spanning Tree Protocol (STP). The Spanning Tree Protocol (STP) is a Layer 2 protocol designed to run on bridges and switches. The spanning tree is a reduction of the layer 2 network mesh constructed such that packets may be forwarded across the network without any looping.
The STP specification is defined in IEEE 802.1D and RSTP (Rapid Spanning Tree Protocol) is defined in IEEE 802.1w (incorporated into IEEE 802.1D-2004). The main objective of STP is to ensure non occurrence of a loop situation, when redundant paths exists in a network. This objective is achieved in two steps. Initially, STP disables network loops by allowing devices to interact with other STP compliant devices in the network to ensure that only one path exists between any two stations on the network. Subsequent to disabling network loops, STP establishes backup links between switches or bridges along the identified non redundant path in a network. Hence, in achieving the aforementioned objective by the STP, a data packet travelling through the network takes a longer path to reach its destination, inspite of an availability of a shorter path.
For the reasons stated above, which will become apparent to those skilled in the art upon reading and understanding the specification, there is a need in the art for a method and a system to avoid loops in the network in an active topology.